The present invention relates to communication techniques, and more particularly to the reception of M-ary phase shift keying modulated signals.
Phase shift keying (PSK) is a well known technique for modulating a carrier signal by a digital information signal. In PSK modulation, binary data representing the information to be communicated is used to control the switching of the phase of a carrier signal between two or more values. For example, in Quaternary PSK (QPSK) modulation, binary information may be taken two bits at a time to control the switching of the phase of a carrier signal between four values, each corresponding to one of the four possible values of the two bits. More generally, the use of PSK to encode M possible values at a time is referred to as M-ary PSK.
PSK systems may be designed that assign each of the possible absolute phase values to one of the possible information values. In a well-known variant, called Differential PSK (DPSK), the system instead responds to changes in phase by comparing a prevailing phase with a preceding phase.
In any of the above PSK-based systems, demodulation of a received (D)PSK-modulated signal requires that the phase of the signal be detected, and that the corresponding informational content be determined. FIG. 1 illustrates a conventional PSK modulation transmitter 101 and receiver 103. On the transmitter side, a PSK modulator 105 generates a PSK-modulated signal, and supplies this to a transmit filter 107. The transmit filter 107 eliminates out of band components of the PSK-modulated signal, and supplies the resultant signal to an digital-to-analog (D/A) converter 109. The analog signal supplied at the output of the D/A converter 109 is supplied to a rst input of a mixer 111. A local oscillator (LO) 113 supplies a carrier frequency to a second input of the mixer 111. The up-converted signal supplied at the output of the mixer 111 is then supplied to a channel 115 (e.g., an air interface) that propagates, the signal to the receiver 103.
In the receiver 103, the signal supplied by the channel 115 is fed to a first input of a mixer 117. A mixer signal supplied by a local oscillator 119 to a second input of the mixer 117 causes an intermediate frequency (IF) signal to be supplied at the output of the mixer 117. The IF signal is supplied to a combined filter 121 that performs two functions. First, the combined filter 121 performs an IF filtering function, represented by the IF filter 123. The IF filter 123 is a selectivity filter whose purpose is to pass the wanted signal without distortion and to reject unwanted out of band signals.
The combined filter 121 also performs a receiver (RX) filtering function, represented by the RX filter 125. The RX filter 125 provides matched filtering to complement that of the transmit filter 107 and may have, for example, a root-cosine response.
The filtered signal generated at the output of the combined filter 121 is supplied to a limiter amplifier 127 (henceforth referred to simply as a “limiter” 127). The limiter 127 changes the generally sinusoidal signal, supplied at its input, into a signal of a repetitive substantially square waveform having two very precisely defined zero crossing points per cycle. The purpose of the limiter 127 is to limit the dynamic range of the signal to be processed. For example, as the distance between the receiver 103 and transmitter 101 varies, the magnitude of the received signal strength may also vary inversely, causing a very wide dynamic range. Use of a limiter 127 is one way of reducing this dynamic range. As an alternative to the limiter 127, an Automatic Gain Control (AGC) circuit could be used for this purpose.
The output of the limiter 127 is then supplied to an analog-to-digital (A/D) converter 129, which generates therefrom a corresponding series of digital values, which are supplied to a PSK demodulator 131. The PSK demodulator 131 uses the supplied digital values to reconstruct the underlying binary information that was transmitted by the transmitter 101.
In conventional systems, all components except for the PSK modulator 105, transmit filter 107 and PSK demodulator 131 are implemented as analog technology. However, in practice the combined filter 121 is difficult to implement as an analog component—its performance deviates from the ideal due to the finite approximation used in realizing the required data shaping (e.g., root raised cosine) and because of variations of the shaped pass-band that result from such factors as manufacturing process, temperature, power supply, matching, frequency offset and the like.
These difficulties could be overcome by using digital rather than analog technology to implement the RX filter 125. It is therefore desirable to provide a (D)PSK receiver in which the functions of the RX filter 121 are performed in the digital domain.